1. Field of the Invention
The present invention relates to electron microscope grids and a process for making the same and also to general applications of the disclosed molding process for fabricating other types of micro-components.
2. Description of the Prior Art
The present invention describes a process of producing very small parts having extremely precise tolerances by a micro-molding process in which the mold is sacrificed after the casting of the part is completed. This micro-molding process is particularly suited for manufacturing specimen supports, such as grids for use in electron microscopy having typical diameters of 3.0 millimeters, typical thicknesses of 1.5 mils (38.1 microns) and line widths of about 150 microns.
In the last decade, electron microscopy has placed increasing emphasis on the use of the electron microscope as an analytical tool to determine the composition and structure of a specimen rather than simply as a device capable of producing a highly magnified image. One analytical method of increasing importance is energy dispersive X-ray analysis. Here, the X-rays emitted from the specimen when bombarded by an electron beam activates a solid state detector and, after being analyzed in a multi-channel analyzer, are displayed in a spectrum where each spectrum peak can be used to identify the presence of a particular element in the specimen.
This procedure is complicated by two factors. First, the electrons are scattered by the specimen and by the interior of the microscope column itself. The electrons can strike the specimen support, generally a fine mesh screen or grid 3 millimeters in diameter as explained earlier, and scatter in various directions. In addition, electrons can strike apertures or any part of the column above the sample producing high energy X-rays which can also strike the grid and be scattered therefrom. Both of these processes cause the grid to produce large amounts of X-ray radiation which produces noise in the spectrum of the multi-channel analyzer.
Since the typical specimen placed on the grid has a thickness of only about 1,000 angstroms while the thickness of the grid itself is approximately 1.5 mils (approximately 380,000 angstroms), the grid is far more massive than the specimen and consequently produces a signal which can in some cases completely mask the detection of trace elements in the specimen. This noise generated by the support has in the past been complicated by impurities within the grid material, especially impurities having high atomic numbers. That is, the noise generated by the support is dependent not only upon the size of the support but also upon the atomic numbers of the various constituent elements forming the material comprising the support. Indeed, the contribution in the energy spectrum due to the grid material is not necessarily limited to frequency peaks at specific frequencies in the spectrum but can also present a "continuum radiation" spread over the entire energy spectrum. The higher the elemental atomic numbers of the material forming the grid, the greater the severity of the continuum radiation interference.
Currently, in electron microscopy, specimen support grids are often made from copper. These grids are inexpensive and readily manufactured via electrolytic deposition or chemical etching. They are generally found to be adequate when only the imaging capabilities are of interest. However, due to the noise factors mentioned earlier, difficulty arises in the use of copper in energy dispersive X-ray analysis. Its high atomic number, 29, results in the generation of noise over a considerable range of the energy spectrum. Thus, since copper is not one of the carbon, hydrogen, oxygen and nitrogen primal elements of organic matter, its use as a specimen support can completely mask the presence of trace elements in the specimen which might easily be detected if the specimen support was also fabricated from a material formed from carbon, hydrogen, oxygen and nitrogen.
Attempts at solving this problem have in the past generally involved the use of grids made from elements with as low an atomic number as possible. Thus, beryllium, a metal with atomic number 4, has been used in fabricating grids. However, beryllium is toxic and is extremely difficult to produce in highly purified form. Consequently, grids made from this material have been quite expensive, costing about five dollars per grid.
Grids made from woven nylon, a material comprising carbon, hydrogen, oxygen and nitrogen, are also commercially available. While the signal to noise ratio in the X-ray spectrum is improved with nylon grids and while such grids are non-toxic, such grids present problems even more severe than beryllium. First such grids are non-conducting and thus cause a problem when bombarded with an electron beam; namely, the generation of electron charge build-up in the vicinity of the grid and specimen which seriously degrades the image quality of the dispersive X-ray analysis equipment. In addition, since the nylon grids when fabricated are stamped out from a woven nylon mesh, they are non-rigid with freedom of deformation in both Cartesian axes of the grid plane. Since the electron beam heats the grid during the X-ray analysis of the specimen, the grid's resultant thermal expansion causes it to deform or creep, thereby preventing a stable specimen image from being observed. Finally, most commercial plastics contain halogen compounds as well as trace amounts of high atomic number elements which are added during the formation of the plastic for purposes of acting as catalysts, hardeners, flame retardants and other similar purposes. All of these additional trace elements further interfere with the energy-dispersive X-ray analysis of the specimen which is generally attempting to discover the presence of trace elements in the specimen.
One proposed solution to the specimen support problem is a grid formed primarily from carbon. Since carbon is a low atomic number element (atomic number 6), is non-toxic and a conductor of heat and electricity, it was believed that making grids from such a material would meet all the structural, heat and electrical conductive requirements of a grid while exhibiting low amounts of X-ray spectral noise. Attempts were made to produce a grid entirely of carbon by sputtering several layers of carbon atop of each other and, in some cases successive pyrolyzing of the deposited carbon. However, it was discovered that grids produced in this fashion were extremely delicate and unable to withstand the mechanical stresses encountered in the normal course of handling during specimen preparation.
The present invention is believed to overcome these deficiencies in prior art specimen support grids for energy dispersive X-ray analysis by being fabricated from a composite material comprising polymers including engineering plastics formed from a combination of some of all of the following organic primal constituent elements--carbon, hydrogen, oxygen and nitrogen--in dispersive mixture with carbon in the range of 10% to 90% by weight.
The composite material is thus formed from the same low atomic number elements ordinarily found in the organic specimens and therefore is non-toxic while generating minimal interfering spectrum noise. The blending of the carbon into the polymer greatly increasing the electrical and thermal conductivity of the composite material thereby minimizing electron charge buildup and thermal expansion of the support and specimen. The blended carbon, especially when in the form of fibers, also adds structural strength to the resultant support.
The process by which these grids are fabricated; namely, the formation of the desired mold by photochemical etching, the casting of the grids followed by dissolving the mold with a substance that does not chemically attack the grids, provides a method by which other extremely small micro-components, such as gears and escapements, can be molded inexpensively without damage or distortion.